The present invention relates to a developing device to be installed in an image forming unit such as a printer and a copy machine, and further to an image forming unit having the developing device and a color image forming apparatus in which a plurality of image forming portions having the developing devices, tandemly arranged.
As the image forming apparatus, conventionally known is an electrophotographic image forming apparatus. The image forming apparatus of this type usually has an image forming unit removably installed therein. The image forming unit performs a series of image forming steps: forming an electrostatic latent image on a photosensitive drum, forming (developing) a toner image from the electrostatic latent image, and transporting the toner image to a toner image transfer position at which the toner image is transferred to a paper sheet from the drum.
FIG. 7A shows a vertical cross-sectional view of the image forming unit of this type. FIG. 7B shows an exploded oblique view of a main portion of the developing device installed, as a subunit, in the image forming unit. FIG. 7C shows a partially enlarged vertical cross-sectional view of the developing device in an assembling step. As shown in FIGS. 7A, 7B and 7C, the image forming unit comprises two subunits, namely, a drum subunit 1 and a developing subunit (device) 2.
The drum subunit 1 includes a photosensitive drum 3, a cleaner 4 positioned in a left-side of the drum, a charging roller 5 positioned in an upper left-side of the drum, and a recording head engaging groove 6 immediately above the drum. The cleaner 4 has a toner scooping sheet 7 and a scratching blade 8 both of which are in pressure-contact with the photosensitive drum 3, as well as a waste toner discharge pipe 9. An optical-writing portion of a recording head (not shown), which is fixed on a main body of the image forming apparatus, is inserted in and engaged with the recording head engaging groove 6.
The photosensitive drum 3 includes a grounded conductive metal roller and a photosensitive layer of a photo-conducting material which is uniformly coated on a circumferential surface of the metal roller. The photosensitive drum 3 rotates in the clockwise direction as indicated by an arrow A in FIG. 7A. The charging roller 5 is connected to a high-voltage supply source and uniformly applies electric charges to the photosensitive drum 3 to initialize the photosensitive layer on the circumferential surface of the drum 3. The photosensitive layer may be charged with positive or negative electric charges. The photosensitive layer of the drum 3 is uniformly charged with, for example, -650V. The recording head has a light source which emits laser beam or light generated by LEDs, and selectively irradiates the negatively charged photosensitive layer of the photosensitive drum 3 in accordance with image data supplied to the recording head from for example a host computer. The irradiated portion of the negatively charged photosensitive layer of the photosensitive drum 3 is decreased in its potential to about -70V. As a result, an electrostatic latent image is formed by the low potential region (-70V) in the initially-charged high-potential region (-650V).
The developing subunit 2 has a casing 10 serving also as a toner hopper. A waste toner collecting bag 11 is provided in an upper interior portion of the casing 10, and a developing roller 12 is provided in a lower opening of the casing 10. The waste toner collecting bag 11 is used for collecting a waste toner discharged from the cleaner 4 by way of the waste toner discharge pipe 9. The developing roller 12 is rotatable with being in pressure-contact with the photosensitive drum 3. The casing 10 is filled with non-magnetic toner 13. A toner stirring member 14 is disposed in a lower interior portion of the casing 10 and is buried in the toner 13. A toner feed roller 15 made of sponge is also provided under the stirring member 14 in the lower interior portion of the casing 10 and is pressed against the developing roller 12. A leaf-spring like doctor blade 16 is further provided in the lower interior portion and is in pressure-contact with the circumferential surface of the developing roller 12 from an upper-right side thereof. A developing-portion scooping sheet 17 made of an insulating film is further disposed in the lower opening of the casing 10 and is in contact with the circumferential surface of the developing roller 12 from a lower side thereof. The scooping sheet 17 is fixed in the lower opening by a press board 19. Sealing members 18a are provided on both sides of the doctor blade 16, and other sealing members 18b are provided on both sides of the developing roller 12. These sealing members 18a, 18b prevent the toner 13 from leaking out from the casing 10 through both sides of the doctor blade 16 and those of the developing roller 12 in the lower opening of the casing 10.
The sealing members 18a, 18b, doctor blade 16, scooping sheet 17 and press board 19 arranged from right to left in FIG. 7B are installed in the lower interior portion of the casing 10 in this order through the lower opening as shown in FIG. 7C. After these members are provided in the lower interior portion of the casing 10, both end portions of the developing roller 12 are inserted in a pair of roller supporting portions 20 in the lower opening of the casing 10 as indicated by an arrow D and are supported by a pair of bearings mounted on the supporting portions 20.
When the both end portions of the developing roller 12 are inserted in the roller supporting portions 20 and supported by the bearings, the developing roller 12 is pressed on the lower sealing members 18b, whereby the lower sealing members 18b come into contact with the developing roller 12. Similarly, the developing roller 12 presses the doctor blade 16 to bent it a predetermined amount, thereby permitting the upper sealing members 18a to come into contact with the doctor blade 16. The photosensitive drum 3 is the widest member of the structural members the image forming unit. The developing roller 12 is smaller in width than the photosensitive drum 3, and a central portion of the developing roller 12 arranged between the both end portions thereof with which the upper sealing members 18a are in contact defines an effective developing region on the circumferential surface of the drum 3. The width of the toner scooping sheet 7 and that of the scratching blade 8 in the cleaner 4 shown in FIG. 7A are shorter than that of the photosensitive drum 3 but longer than that of the developing roller 12.
In this structure, the toner stirring member 14 of the developing subunit 2 (shown in FIG. 7A) rotates to stir the toner 13 and to feed the toner 13 toward the feed roller 15 positioned below the stirring member 14. A feed bias voltage of, for example, -500V is applied to the feed roller 15 by a bias power supply, and the feed roller 15 rotates in the counterclockwise direction as indicated by an arrow C in the figure while the feed roller 15 presses its sponge surface on the developing roller 12. Therefore, the feed roller 15 supplies the toner 13 to the circumferential surface of the developing roller 12 in a rubbing manner. A developing bias of -250V is applied to the developing roller 12 by a bias power supply, and the developing roller 12 rotates in the counterclockwise direction as indicated by an arrow B in the figure. The doctor blade 16, which is appropriately pressed against the circumferential surface of the developing roller 12, gives a weak negative triboelectric charge to the toner 13 to assist an attraction of the toner 13 onto the developing roller 12, and controls a thickness of a toner layer attracted on the developing roller 12 in a predetermined value. The developing roller 12 transports the toner layer by its rotation to a position (developing portion) facing the photosensitive drum 3.
At the position where the developing roller 12 faces the photosensitive drum 3 of the drum subunit 1, a potential difference of -180V is produced between the low potential region (-70V), that is the statistic latent image, formed on the photosensitive drum 3 and the developing roller 12 (-250V). This means that the low potential region (the electrostatic latent image) is positively charged relative to the developing roller 12. This causes the negatively charged nonmagnetic toner 13 to transfer from the developing roller 12 to the relatively positively-charged low potential region (electrostatic latent image) formed on the photosensitive drum 3 and to form a toner image (reversal development). The toner image developed from the latent image on the circumferential surface of the photosensitive drum 3 is transported by the rotation of the drum 3 to a lower end of the photosensitive drum 3. Since a paper sheet (not shown) moved from right to left in the figure contacts the lower end of the photosensitive drum 3, the toner image on the circumferential surface of the photosensitive drum 3 is transferred onto the paper sheet and then thermally fixed thereon by a fixing device (not shown).
After completion of the image transfer onto the paper sheet, a small amount of toner which can not be transferred remains on the circumferential surface of the photosensitive drum 3. The remaining toner on the drum is passed under the cleaner scooping sheet 7 and is introduced into the cleaner 4. Finally, the remaining toner is scratched off from the photosensitive drum 3 by the scratching blade 8, and the cleaner scooping sheet 7 scoops the scratched-off toner to prevent it from leaking out from the cleaner 4.
The developing device of the conventional image forming unit mentioned above has a following problem. When a solid image having a large area is developed, a density gradation often appears in a printed solid image on a paper sheet. The density gradation is produced by a first part of the solid image developed by a first round of the developing roller 12 and a second part or the following parts of the solid image developed by a second round or the following rounds thereof. This is, until a development of a solid latent image by the developing roller 12 is started, the developing roller 12 has made a plurality of rotations so that the feed roller 15 also has made a plurality of rotations to supply the toner to the roller 12 many times and the doctor blade 16 has given triboelectric charges to the roller 12 many times, thereby increasing the density of the toner layer on the roller 12. As a result of this, the developing roller 12 develops the solid image on the photosensitive drum 3 with the high-density toner layer during the first rotation of the roller 12, and the toner is scarcely left on the developing roller 12. In the second rotation and the following rotations of the developing roller 12 during the development of the drum 3, the toner layer is formed on the roller 12 by only one supply of the toner from the supply roller 15 and only one give of the triboelectric charge to the toner layer on the roller 12 from the doctor blade 16. The new toner layer formed on the roller 12 by one toner supply and one triboelectric-charging is low in toner density compared to the toner layer formed by a plurality of toner supplies and a plurality of triboelectric-charging. This is the reason, why the density gradation (hereinafter referred to as "development memory") as mentioned above takes place.
This problem (the phenomenon of the development memory) may be overcome if the toner deposited on the circumferential surface of the developing roller 12 is completely removed just after the circumferential surface of the developing roller 12 is separated from the developing portion at which the roller 12 contacts the photosensitive drum 3, and then the toner 13 is newly supplied to the roller 12 in the casing 10. If so, the toner layer is always formed on the roller 12 in every one rotation of the roller 12 by one toner supply and one triboelectric-charging. Consequently, the phenomenon of the development memory should be overcome. In the conventional image forming unit mentioned above, the rotational direction of the feed roller 15 is opposite to that of the developing roller 12 at a toner feed portion at which the feed roller 15 contacts the developing roller 12, and the feed roller 15 rubs the developing roller 12 at the toner feed portion not only to feed the toner onto the developing roller 12 but also to remove the residual toner, which was not used to develop the latent image on the drum 3, from the roller 12.
However, the phenomenon of the development memory has not yet been overcome by the aforementioned structure of the conventional image forming unit. To overcome the phenomenon of the development memory, it is known to set a scratching member, which serves exclusively as a toner scraper, to abut against the circumferential surface of the developing roller 12. However, this scratching member makes the overall size of the developing subunit increase. In addition, this scratching member further makes the driving force of the developing roller 12 larger, and hence the developing subunit requires a large motor and a strong driving force transmitting system. These large motor and the strong driving force transmitting system further makes the developing subunit inevitably increase its overall size and its manufacturing cost.